Display devices such as CRT's, LCD's, LED's and gas discharge displays use electron beams, electrostatic fields and/or magnetic fields for the purpose of displaying graphic images, information and data. Such display devices operate on static and dynamic electrical fields, i.e. electrical voltages and currents, which as a by-product emit time-varying magnetic fields. Such magnetic fields create harmful biological effects in humans, and interfere with radio transmissions and other devices in the vicinity of the display devices which employ electromagnetic waves.
Since the strength of a magnetic field decreases with distance, it has been suggested that the harmful biological effects of the magnetic field emitted from display devices can be avoided by maintaining an adequate, safe distance from such devices, such as one meter. Also, since the harmful effects of magnetic fields increase with exposure, it has been suggested that the harmful effects can be reduced by turning off the display devices when not in use.
Unfortunately, many display devices must be in use for substantial lengths of time making it unfeasible to reduce emissions by turning off the device. Further, due to the small character size displayed on most of such devices, it is impractical to maintain an adequate safe distance from the device.
In addition to the magnetic fields emitted from the display devices, there exist ambient magnetic fields produced by the earth and other electrical devices. These ambient magnetic fields can interfere with the proper operation of the above-mentioned display devices by altering the color display and shifting the raster beam.
Numerous techniques have been developed to block the introduction of external ambient magnetic fields into such display devices to prevent interference with their normal operation. However, such magnetic cancellation devices tend to increase the total magnetic field emitted from the display device, thereby increasing the interference with other devices in the vicinity and increasing the harmful biological effects caused by the magnetic field. For example, U.S. Pat. No. 5,066,891 to Harrold et al. discloses a magnetic field sensing and cancelling circuit for use with a CRT which comprises a closed loop system of flux gate sensors to measure external magnetic fields and magnetic field cancellation coils disposed around the perimeter of the face plate of the CRT to cancel the measured field and thus minimize color and positional image distortion. The cancellation coils, however, emit an outwardly directed magnetic field to cancel the measured field, thereby increasing the magnetic field directed at the user.
For the foregoing reasons, there is a need to develop an apparatus and method to reduce the intensity of the magnetic field emitted from display devices.